1. Technical Field
The following description relates to one or more sheet feeders configured to feed sheets separately on a sheet-by-sheet basis and smoothly to a scanner unit or a printer unit.
2. Related Art
As a sheet feeder, employed for an image scanner or a printer, which is configured to feed sheets stacked on a feed tray to a scanner unit or a printer unit separately on a sheet-by-sheet basis, Japanese Utility Model Provisional Publication No. SHO59-159651 discloses an Automatic Document Feeder (ADF) that includes a feed tray configured to be loaded with plural document sheets thereon, a feed roller (a separation roller) that is provided at a downstream side in a sheet feeding direction relative to the feed tray and configured to rotate in the sheet feeding direction, a high-friction discrimination arm configured to contact the feed roller, a low-friction film that is provided on a side of a contact surface between the discrimination arm and the feed roller and formed to become narrower in the sheet feeding direction, and an urging member configured to urge the discrimination arm toward the feed roller.
In the above configuration, now the following situation is assumed: easily-bendable document sheets are stacked high on the feed tray and a leading end of a sheet among the stack of document sheets that is at the closest level to a circumferential surface of the feed roller is fed in a tangential direction of the circumferential surface of the feed roller and then conveyed toward a surface of the discrimination arm. In this situation, when the film is short and a leading end thereof is far at an upstream side in the sheet feeding direction away from a nipping point between the feed roller and the discrimination arm, the leading end of the document sheet cannot establish sliding contact with a surface of the film at a small angle. Consequently, the leading end of the document sheet collides directly against the high-friction surface of the discrimination arm at an angle as large as 60 degrees. It results in great resistance to feeding the leading end of the document sheet in the sheet feeding direction. Nevertheless, a portion of the document sheet on an upstream side in the sheet feeding direction relative to the leading end thereof is forced to be fed to the downstream side by the circumferential surface of the feed roller. Therefore, the upstream-side portion of the sheet is reformed in a curled shape. A range in which the reformation of the sheet into a curled shape is caused corresponds substantially to a width of the discrimination arm (i.e., a length of the discrimination arm in a direction parallel to a surface of the sheet and perpendicular to the sheet feeding direction). Thus, the reformation of the sheet is regarded as a kind of buckling.
On the contrary, when the leading end of the film is extended close to the nipping point between the feed roller and the discrimination arm, an effective high-friction area on the surface of the discrimination arm is reduced. Consequently, the reformation of the sheet into a curled shape is hardly caused. However, when the leading end of the film reaches the nipping point, an operation of separating sheets cannot be achieved between the feed roller and the discrimination arm, and it results in multiple-sheet feeding. Nonetheless, a positional relationship between the feed roller and the discrimination arm that may vary within a production tolerance unfortunately makes it difficult to locate the leading end of the film in an appropriate position.
Japanese Patent Provisional Publication No. HEI6-227693 (see FIGS. 1 to 6 in the Publication) discloses a sheet feeder that includes, at an end on a sheet feeding side, a rubber feed roller driven to rotate and a high-friction separation arm configured to contact an upper circumferential surface of the feed roller. The sheet feeder further includes a metal plate regulator disposed such that a distal end thereof presses a portion, of a back surface of the separation arm, near a nipping point between the separation arm and an upper circumferential surface of the feed roller. Thus, a gap between the feed roller and the separation arm is regulated to a distance such that a single sheet passes therethrough.
Further, the sheet feeder includes lower spring members disposed at both sides in a width direction of the separation arm. Bases of the lower spring members formed from metal plates are fixed to a bracket together with the separation arm. The bases of the lower spring members are fixed to a surface of the separation arm that faces the circumferential surface of the feed roller. Furthermore, free ends of the lower spring members are extended up to such a position, at an upstream side in the sheet feeding direction relative to the nipping point, as to nearly contact the upper circumferential surface of the feed roller. In this configuration, sheets of a stack of sheets on a feed tray are inserted into between the upper circumferential surface of the feed roller and the lower spring members in an overlapping state. At this time, the multiple-sheet feeding can allegedly be prevented by such an operation that a sheet, which is in contact with the upper circumferential surface of the feed roller, proceeds ahead of a sheet which is receiving high frictional resistance while contacting the separation arm. The configuration has a problem that the number of sheets settable on the feed tray is limited to as many as ten.
On the other hand, Japanese Patent Publication No. 3567609 (hereinafter, simply referred to as '609 Publication) (see FIGS. 1 and 2 in '609 Publication) and FIGS. 14A and 14B disclose a document feeder that includes a separation arm 101, a first plate spring 103, and a second plate spring 102. A back surface side of the separation arm 101 is supported by the first plate spring 103 which is configured to be elastic. An engagement portion 104 bent to get up from a base of the first plate spring 103 is engaged with an engagement hole 105 provided at a base of the separation arm 101. Thereby, a surface of a distal end side of the separation arm 101 is pressed by a circumferential surface of a feed roller 100.
A base of the second plate spring made of metal is fixed to a housing such as a cover member, together with a base of the first plate spring. The second plate spring 102 has two-forked distal end portions that extend from the base of the second plate spring 102 along both sides in a width direction of the separation arm 101. Each of the two-forked distal end portions of the second plate spring 102 has a distal end bending portion 102a formed to bend in a V-shape when laterally viewed. The length of the feed roller 100 in the axial direction thereof is larger than the distance between the two distal end bending portions 102a. When laterally viewed, the two distal end bending portions 102a are located between a position 108 where a leading end of a top sheet of sheets 106 stacked at an upstream side in the sheet feeding direction contacts the circumferential surface of the feed roller 100 and a position 107 where the separation arm 101 contacts the circumferential surface of the feed roller 100 (i.e., a nipping point where a sheet is nipped between the separation arm 101 and the circumferential surface of the feed roller 100. Further, the distal end bending portions 102a are disposed to be in contact with the circumferential surface of the feed roller 100 or as close thereto as possible. According to the above configuration, since an angle θ between the separation arm 101 and the sheets 106 can be set larger, the distal end bending portions 102a can be shifted to a downstream side in the sheet feeding direction relatively in comparison with a conventional configuration. Further, as an angle φ between the distal end bending portions 102a and the sheets 106 is set large, the sheets 106 are allegedly separated more certainly so as to avoid a problem such as the multiple-sheet feeding and no-sheet feeding.